Tall oil materials are used as starting materials in the chemical industry, for example in paints, coatings, adhesives, printing inks, paper chemicals and in the manufacture of hydrocarbons, suitable as liquid fuels, as transportation fuels, or compatible with transportation fuels. Biofuels are typically manufactured from starting materials originating from renewable sources including tall oil materials, such as crude tall oil. The starting material can be hydroprocessed in one or more steps to yield hydrocarbons.
Tall oil materials, such as crude tall oil, are mixtures of a wide range of various compounds. Crude tall oil is obtained as a by-product from the pulp and paper industry. In the pulping process “black liquor” obtained from pulp cooking is concentrated and allowed to settle. The top layer known as “tall oil soap” is then skimmed off. Tall oil soap is reacted with an acid, and typically this acidulation is carried out with sulphuric acid, whereby crude tall oil is formed and sulphur containing gases, such as hydrogen sulphide, methylmercaptan, dimethyl sulphide and dimethyldisulphide are released. The acidulation step in the pulp mill is often inaccurate, whereby varying amounts of tall oil soap remain in the crude tall oil because of incomplete acidulation. Problems relating to corrosion due to the acidic conditions and increased odorous gas emissions, as well as of the aim to reduce costs and load of sulphate salts are often reasons why pulp mills tend to use as little as possible of sulphuric acid.
Crude tall oil (CTO) is renewable material, which comprises a mixture of plant derived compounds obtained as a by-product from processes of the forest industry, typically from processing of pine and/or spruce trees. CTO is mainly composed of both saturated and unsaturated oxygen-containing organic compounds, such as unsaponifiable matter, neutral substances including sterols and esters, resin acids (mainly abietic acid and its isomers), fatty acids (mainly palmitic acid, linoleic acid, oleic acid and linolenic acid), fatty alcohols, sterols and other alkyl hydrocarbon derivatives. Typically, CTO contains varying amounts of impurities, such as inorganic sulphur compounds, metals, Na, K, Ca and phosphorus, tall oil soap and water. The composition of CTO and the amounts of impurities therein vary depending on the specific wood species and on the process yielding CTO.
The impurities in CTO refer here particularly to metallic impurities, impurities comprising Na, K, Ca, P, tall oil soap and water, and combinations thereof.
Wood type has obvious effect on the quality and composition of the produced crude tall oil, and different proportions of resin acids, fatty acids and unsaponifiables are produced from different wood species.
Tall oil soap contains accumulated metallic impurities and impurities comprising any of Na, K, Ca and P, which act as catalyst inhibitors and poisons in possible subsequent catalytic processing. Tall oil soap contains also some water. Crude tall oil used as starting material in hydroprocessing should contain as little as possible of these impurities.
Crude tall oil may be purified by means of washing with washing liquid, filtering, distillation, degumming, depitching and evaporating. Typically, purification by depitching is used where nonvolatile materials are removed, or alternatively distillation. However, simultaneously part of valuable starting material is lost. Further, crude tall oil is sensitive to heat, which may cause degradation and subsequent loss of product.
U.S. 2011213169 A1 discloses a method and apparatus for purifying CTO, where metals and other impurities are removed from CTO by precipitation with acidification.
WO 2009131510 A1 describes a method for converting CTO to diesel fuel, said method comprising the step where residual soap and impurities are removed from CTO by washing with sulphuric acid.
WO 2013173077 A2 relates to a semi-continuous acidulation of CTO soap, where CTO soap is combined with sulphuric acid and water.
WO 2015055896 A1 discloses pretreatment of CTO for removing impurities, where the pretreatment comprises at least two evaporative steps.
WO 2014098692 A1 relates to removal of impurities from CTO by washing with water containing chelating agent.
CA 2133663 C describes reduction of residual soap in CTO by using strong acids.
Despite the ongoing research and development of processes utilizing crude tall oil as feedstock, there is still a need to provide an improved method for reducing impurities, particularly residual soap in crude tall oil.